It is preferable to design an engine for a constant load so that a desired torque may generate at a low specific fuel consumption. However, it is difficult to design an engine for driving vehicles so as to have low specific fuel consumption within the entire range of the engine operation, since load on the engine varies in a wide range.
FIG. 1 shows a fuel consumption characteristic of an engine for a vehicle at various specific fuel consumptions (g/ps.hr), in which abscissa is engine speed (r.p.m.), ordinate is engine torque. Curve A shows running load (resistance) of a vehicle on a flat road. The curve A is decided by drag of the body of the vehicle and gear ratio of the transmission of the engine and the specific fuel consumption is decided by the performance of the engine. It is desirable to design the engine so that the curve A may pass through low fuel consumption zones.